1. Field of the Invention
This invention relates to a distance measurement apparatus using an electromagnetic wave modulated in accordance with a pseudo random noise code.
2. Description of the Related Art
A prior-art distance measurement apparatus of a spread spectrum type which is mounted on an automotive vehicle measures the distance between the present vehicle and a preceding target object such as a preceding vehicle by using an electromagnetic wave modulated in accordance with a pseudo random noise code (a PN code). Specifically, a beam of an electromagnetic wave whose amplitude is modulated in accordance with a PN code of a predetermined bit length (a predetermined chip length) is emitted in a forward direction with respect to the body of the present vehicle. A moment of the transmission of the PN code with the electromagnetic wave is memorized. The prior-art apparatus receives an echo beam caused by reflection of the forward electromagnetic-wave beam at a preceding target object. The received echo beam is converted into a corresponding electric signal. The echo-beam-corresponding electric signal is binarized into a bi-level echo electric signal. Calculation is made about the value of the correlation between the bi-level echo electric signal and the PN code used for the modulation of the transmitted electromagnetic wave. A moment at which the calculated correlation value peaks is detected as a moment of the reception of the PN code contained in the echo beam. The prior-art apparatus calculates the distance between the present vehicle and the preceding target object from the time interval between the moment of the transmission of the PN code and the moment of the reception thereof, and also from the velocity of the electromagnetic wave.
An example of the electromagnetic wave is light emitted from a laser diode. In general, the life of a laser diode is inversely proportional to the second power to the fourth power of the total time for which the laser diode is active.
Successive bits (chips) of xe2x80x9c1xe2x80x9d and successive bits of xe2x80x9c0xe2x80x9d in the PN code cause the PN-code frequency spectrum to extend into a low-frequency side. Such a spectrum extension into a low-frequency side tends to fluctuate the echo-beam-corresponding electric signal at a low frequency. The low-frequency fluctuation reduces the accuracy of binarizing the echo-beam-corresponding electric signal into the bi-level echo electric signal, and also the accuracy of the calculated correlation between the bi-level echo electric signal and the PN code used for the modulation of the transmitted electromagnetic wave.
U.S. Pat. No. 6,218,982 B1 discloses a distance measurement apparatus in which a pseudo random noise code is generated synchronously with a reference clock signal. A first forward electromagnetic wave is transmitted in response to the pseudo random noise code. A first echo wave is received which is caused by reflection of the first forward electromagnetic wave at an object. The received first echo wave is converted into a binary signal. A value of a correlation between the binary signal and the pseudo random noise code is repetitively calculated at a predetermined period having a synchronous relation with the reference clock signal. A time interval taken by the first forward electromagnetic wave and the first echo wave to travel to and from the object is measured in response to a timing at which the calculated correlation value peaks. Then, a second forward electromagnetic wave is transmitted in response to a transmitted pulse signal. A second echo wave related to the second forward electromagnetic wave is received. The received second echo wave is converted into a received pulse signal. A delay circuit defers the transmitted pulse signal by a delay time corresponding to the measured time interval to generate a delayed transmitted pulse signal. A phase difference between the received pulse signal and the delayed transmitted pulse signal is measured at a resolution higher than a resolution corresponding to the predetermined period of the correlation-value calculation. A distance to the object is calculated on the basis of the measured time interval and the measured phase difference.
U.S. patent application Ser. No. 09/429,164, filed on Oct. 28, 1999, relates to a distance measurement apparatus which includes a transmitting device for transmitting a forward electromagnetic wave. A receiving device operates for receiving an echo wave caused by reflection of the forward electromagnetic wave at an object, and converting the received echo wave into a first received signal. A scattered-wave detecting device operates for detecting a scattered wave reaching the receiving device and caused by reflection of the forward electromagnetic wave at an obstacle which occurs prior to the reflection of the forward electromagnetic wave at the object. A cancel-signal generating device operates for generating a cancel signal to cancel a scattered-wave-corresponding component of the first received signal generated by the receiving device in response to the scattered wave detected by the scattered-wave detecting device. A scattered-wave canceling device operates for removing the scattered-wave-corresponding component from the first received signal in response to the cancel signal generated by the cancel-signal generating device to change the first received signal to a second received signal. A distance calculating device responsive to the second received signal operates for measuring a time interval between a moment of the transmission of the forward electromagnetic wave by the transmitting device and a moment of the reception of the echo wave by the receiving device, and calculating a distance to the object on the basis of the measured time interval.
Japanese patent application publication number 60-102015 discloses first and second apparatuses each for generating a pseudo random noise code (a PN code). The first apparatus in Japanese application 60-102015 includes a feedback circuit having an n-stage shift register and half adders. The n-stage shift register and the half adders are connected in a closed loop. The half adders are assigned respectively to selected ones among the stages of the n-stage shift register. The first half adder executes half addition between the output signal from the final stage of the n-stage shift register and the output signal from the assigned stage of the n-stage shift register which precedes the final stage thereof. Each of the second and later half adders executes half addition between the output signal from the immediately-preceding half adder and the output signal from the assigned stage of the n-stage shift register. The output signal from the final half adder is inputted into the first stage of the n-stage shift register. An (m+1)-stage shift register is connected to the output terminal of the n-stage shift register. The (m+1)-stage shift register is composed of D flip-flops connected in cascade. An AND circuit executes AND operation among the output signals from the D flip-flops. The output signal from the AND circuit is applied to a first input terminal of an OR circuit. A reset signal is applied to a second input terminal of the OR circuit. The reset signal is also fed to the stages of the n-stage shift register. The output terminal of the OR circuit is connected with the reset terminals of the D flip-flops. The n-stage shift register and the (m+1)-stage shift register are driven by a common clock signal. The output signal from the first stage of the (m+1)-stage shift register is used as an output PN code. When all the output signals from the stages of the (m+1)-stage shift register become xe2x80x9c1xe2x80x9d, the AND circuit resets the D flip-flops to states corresponding to xe2x80x9c0xe2x80x9d. Accordingly, in the output PN code, a bit (a chip) immediately following xe2x80x9cmxe2x80x9d successive bits (chips) of xe2x80x9c1xe2x80x9d is always set to xe2x80x9c0xe2x80x9d. In other words, xe2x80x9cmxe2x80x9d successive bits of xe2x80x9c1xe2x80x9d may occur while (m+1) successive bits of xe2x80x9c1xe2x80x9d are inhibited from occurring. The first D flip-flop acts an inverter for changing a bit of xe2x80x9c1xe2x80x9d to a bit of xe2x80x9c0xe2x80x9d.
The second apparatus in Japanese application 60-102015 is improved over the first apparatus therein. The second apparatus in Japanese application 60-102015 detects not only when all the output signals from the stages of the (m+1)-stage shift register become xe2x80x9c1xe2x80x9d but also when all the output signals from the stages of the (m+1)-stage shift register become xe2x80x9c0xe2x80x9d. The detection of successive bits (chips) of xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d is used to implement the following operation steps. In the output PN code, a bit immediately following xe2x80x9cmxe2x80x9d successive bits of xe2x80x9c1xe2x80x9d is always set to xe2x80x9c0xe2x80x9d. On the other hand, a bit immediately following xe2x80x9cmxe2x80x9d successive bits of xe2x80x9c0xe2x80x9d is always set to xe2x80x9c1xe2x80x9d. Thus, in the output PN code, the number of successive bits of xe2x80x9c1xe2x80x9d and also the number of successive bits of xe2x80x9c0xe2x80x9d are limited to xe2x80x9cmxe2x80x9d or less.
Japanese patent application publication number 7-321611 discloses a PN code generation circuit including a counter, a ROM, a code length detector, and a switch. The ROM is divided into segments which store different n-bit (n-chip) PN code signals respectively. Here, xe2x80x9cnxe2x80x9d denotes a predetermined number indicating a PN code length. In each of the ROM segments, xe2x80x9cnxe2x80x9d bits (chips) composing the related PN code signal are located at different addresses respectively. The counter counts every pulse of a clock signal, and outputs a signal representing the counted pulse number. The code length detector responds to the output signal from the counter. Specifically, the code length detector outputs a reset signal to the counter when the counted pulse number represented by the output signal from the counter reaches the PN code length number xe2x80x9cnxe2x80x9d. The output signal from the counter is fed to the ROM segments as an address signal. Accordingly, the ROM segments output the PN code signals respectively. The switch which follows the ROM segments selects one among the PN code signals as a final PN code signal.
It is an object of this invention to provide an improved distance measurement apparatus using an electromagnetic wave modulated in accordance with a pseudo random noise code.
A first aspect of this invention provides a distance measurement apparatus comprising electromagnetic wave generating means for generating and transmitting an electromagnetic wave; code generating means for generating a pseudo random noise code having a sequence of a predetermined number of chips each being in one of first and second logic states different from one another; code monitoring means for detecting at least a preset number of successive chips in the first logic state in the pseudo random noise code generated by the code generating means, and for changing at least one among the detected preset number of successive chips to the second logic state to convert the pseudo random noise code into a transmission signal; driving means for activating and deactivating the electromagnetic wave generating means to modulate the electromagnetic wave in response to the transmission signal generated by the code monitoring means; receiving means for receiving an echo wave caused by reflection of the electromagnetic wave generated by the electromagnetic wave generating means and modulated by the driving means at an object, and for converting the received echo wave into a received signal; correlation calculating means for calculating a correlation between the received signal generated by the receiving means and the transmission signal used by the driving means; and distance calculating means for detecting a moment at which the correlation calculated by the correlation calculating means peaks, and for calculating a distance to the object from the detected moment.
A second aspect of this invention is based on the first aspect thereof, and provides a distance measurement apparatus wherein the electromagnetic wave generating means includes a light emitting element, and the driving means activates the light emitting element to allow generation of light when the transmission signal assumes a logic state corresponding to the first logic state, and deactivates the light emitting element to inhibit generation of the light when the transmission signal assumes a logic state corresponding to the second logic state.
A third aspect of this invention is based on the first aspect thereof, and provides a distance measurement apparatus wherein the driving means activates the electromagnetic wave generating means to allow generation of the electromagnetic wave when the transmission signal assumes a logic state corresponding to the first logic state, and deactivates the electromagnetic wave generating means to inhibit generation of the electromagnetic wave when the transmission signal assumes a logic state corresponding to the second logic state.
A fourth aspect of this invention is based on the first aspect thereof, and provides a distance measurement apparatus wherein the code monitoring means comprises means for detecting at least a given number of successive chips in the second logic state in the pseudo random noise code generated by the code generating means, and for changing at least one among the detected given number of successive chips to the first logic state to convert the pseudo random noise code into the transmission signal.
A fifth aspect of this invention provides a distance measurement apparatus comprising first means for generating a first pseudo random noise code having a sequence of chips each being in one of first and second logic states different from one another; second means for detecting a preset number of successive chips in the first logic state in the first pseudo random noise code generated by the first means, and for changing last one among the detected preset number of successive chips to the second logic state to convert the first pseudo random noise code into a second pseudo random noise code; and third means for emitting an electromagnetic wave in response to the second pseudo random noise code generated by the second means.
A sixth aspect of this invention provides a distance measurement apparatus comprising first means for generating a first pseudo random noise code having a sequence of chips each being in one of first and second logic states different from one another; second means for detecting a preset number of successive chips in the first logic state in the first pseudo random noise code generated by the first means, and for changing last one among the detected preset number of successive chips to the second logic state to convert the first pseudo random noise code into a second pseudo random noise code; third means for detecting a given number of successive chips in the second logic state in the second pseudo random noise code generated by the second means, and for changing last one among the detected given number of successive chips to the first logic state to convert the second pseudo random noise code into a third pseudo random noise code; and fourth means for emitting an electromagnetic wave in response to the third pseudo random noise code generated by the third means.